Coronary artery atherosclerosis disease (CAD) is the leading cause of death and disability in Western society and is expected to be the number one cause of death worldwide by 2020 ((Topol, et al., 2006 Hum. Mol. Genet., 15 Spec No 2:R117-23). It affects more than 11 million people in the United States alone. Occlusion of coronary vessels results in reduced blood flow to heart muscle, damage to this tissue and ultimately myocardial infarction (MI), of which there are over 1.5 million in the United States each year.
Treatment of CAD in the last two decades has progressively shifted from a predominantly surgical approach to the increasing use of percutaneously introduced therapies (Wong, et al., Metab Syndr Relat Disord. 2006 Winter; 4(4):233-6. Percutaneous transluminal coronary balloon angioplasty (PTCA) involves the insertion of a balloon catheter into the peripheral circulation, which is threaded through the arterial system into the affected vessel. Inflation of the balloon expands the lumen of the diseased artery and pushes back obstructing plaques, effectively increasing the blood flow in this area. The therapy, however, does not usually result in a permanent opening of the affected coronary artery and can cause significant damage to the treated vessel.
A key problem associated with PTCA is the injury caused to the artery during inflation of the balloon and the corresponding hyperextension of the vessel wall. Following deflation and removal of the balloon, damaged portions of the arterial lining can occlude the vessel and thrombosis can develop as a result of damage to the endothelial cell lining. More commonly though, recoil and remodeling of the ballooned vessel triggers the hyper proliferation of smooth muscle cells leading to neointima formation and narrowing of the vessel lumen. This narrowing is commonly referred to restenosis (Scott, 2006 Adv. Drug Del.iv Rev., 58(3):358-76 (2006)). Two key clinical trials (BENESTENT, STRESS) in the early 1990s (Serruys, et al., N. Engl. J. Med., 331:489-95 (1994); Fischman, et al., N. Engl. J. Med., 331:496-501 (1994)) established the superiority of stent placement over PCTA alone. They concluded that the clinical and angiographic outcomes were better in patients who received a stent than in those who received standard coronary angioplasty and subsequent reintervention was required less often (Ozaki, et al., Prog. Cardiovasc. Dis., 39:129-40 (1996)). It was found that the use of coronary stents was effective at inhibiting both vessel recoil and remodeling, decreasing the frequency of restenosis by approximately 50% (Hoffman and Mintz, Eur. Heart Jour., 21:1739-49 (2000)). Stents are metal scaffolds, commonly stainless steel or an alloy such as cobalt-chromium and are usually cylindrical or tubular in shape. Their primary function is to hold open the treated artery following lumen expansion, preventing recoil and minimizing damage to the vessel. Stents can be crimped onto a catheter and balloon, such as in PCTA or be self expanding. By providing scaffolding upon expansion, stents are able to increase the size of the arterial lumen to a greater extent than PTCA by preventing arterial recoil following balloon dilatation.
While stents reduce the problem of vessel recoil, the excessive proliferation of smooth muscle cells remains problematic. Stents induce proliferation at both the luminal and tissue (mural) surfaces and trigger inflammation concentrated at the stent struts. The occurrence and extent of in-stent restenosis is correlated to the amount of physical damage to the endothelium and severity of the disruption to adventia and media vessel layers that occurs upon deployment (Toutouzas, et al., Eur. Heart Jour., 25:1679-89 (2004)).
Metal alloys are widely used in medicine, with stainless steel, titanium, cobalt-chromium and nitinol most commonly used (Singh and Dahotre, J. Mater. Sci.: Mater. Med., 18(5):725-51 (2007)). In cardiovascular medicine metallic implants generally provide blood contacting surfaces such as those used in replacement heart valves, pacemaker components, hemodialysis membranes, and stents (Balasubramanian, et al., J. Biomater. Sci. Polymer Edn., 9(12):1349-59 (1998)). These materials provide favourable tensile properties, fatigue and corrosion resistance, but suffer from poor endothelial cell interactions and are profoundly thrombogenic (Hong, et al., Biomaterials, 26(26):5359-67 (2005)). The positive aspects of stent use are also counter balanced by the thrombogenic nature of stainless steel (and other stent materials) and thus the requirement for anti-platelet therapy. Despite the re-narrowing of stented vessels and the common need for reintervention, the use of stents has increased, such that by 1999 almost 85% of percutaneous coronary interventions involved their use (Serruys, et al., N. Engl. J. Med., 354:483-95 (2006)).
The most favoured approach to improving stent performance has been to coat the stents with various anti-thrombogenic or anti-proliferative agents in order to reduce thrombosis and restenosis. The two most successful anti-proliferative compounds used for drug eluting stent (“DES”), Sirolimus (and its limus variants) and Paclitaxel, interrupt smooth muscle cell proliferation and endothelialisation, preventing occlusion of the vessel caused by neointimal hyperplasia (Gershlick, Heart, 91 Suppl 3:24-31 (2005)). The drugs are generally delivered from nonerodible or erodible polymer layers covering the stent and can be released in a range of doses and rates. Both have been shown to be highly effective at reducing restenosis and re-intervention rates, when compared to bare metal stents. The overall rate of adverse cardiac events was also reduced, in the case of Sirolimus, by about 25%, due entirely to the increased revascularization achieved when using a DES (Moses, et al., N. Engl. J. Med., 349:1315-23 (2003); Morice, et al., N. Engl. J. Med., 346:1773-80 (2002)). Current trial data indicates that there are not significant differences in the performance of these two anti-proliferative agents, and both are equally effective (Brodie, Jour. Intervent. Cardiol., 19:39-42 (2006)).
Due to the high efficacy demonstrated for these compounds, the use of DES has markedly increased over the five years they have been available and are now in widespread use. Only recently have clinicians concluded long-term follow ups on patients receiving DES and some are signalling that not all the effects of DES are positive (Tsimikas, Jour. Amer. Coll. Cardiol., 47:2112-5 (2006)). Concerns regarding late stent thrombosis in DES were first raised in 2003/4 and are being perpetuated by longer term clinical studies in 2006.
One such recent study (BASKET-LATE) prospectively followed 746 consecutive patients who randomly received either DES or bare-metal stents in the Basel Stent Kosten-Effektivitats Trial (Kaiser, et al., Lancet, 366:921-9 (2005)) and who were event-free at the time their anti-platelet therapy (Clopidogrel) was stopped (six months). For BASKET-LATE, the patients were followed for an additional year beyond the six-month mark. Following the discontinuation of anti-platelet therapy, the rates of MI were higher in those patients receiving DES than those with bare-metal implants (4.9% vs 1.3%). DES was also associated with increased risk of death, possibly caused by late thrombosis formation (Pfisterer, et al., Jour. Amer. Coll. Cardiol., 48:2584-91 (2006)).
Materials exhibiting true biocompatibility perform their function without eliciting an undue host response or resulting in adverse clinical outcomes (Williams, Biomaterials, 29(20):2941-53 (2008)). Further, truly biocompatible materials should facilitate beneficial cell and tissue interactions appropriate to the specific application. A key problem with respect to cell interactions of endovascular stents has been suboptimal endothelialisation. This is highlighted by drug-eluting coronary stents where delay and/or absence of stent endothelialisation, predisposes to late thrombotic events with adverse clinical consequences (Pfisterer, et al., Jour. Amer. Coll. Cardiol., 48:2584-91 (2006)).
Efforts to enhance the biocompatibility of metal surfaces have involved a number of diverse approaches. Heat treatment (Plant, et al., Biomaterials, 26(26):5359-67 (2005)), surface oxidation (passivation) (Prunotto and Galloni, Anal. Bioanal. Chem., 381(3):531-3 (2005)), nano-scale patterning (Lu, et al., Acta Biomater., 4(1):192-210 (2008)) and carbon coatings (Ma, et al., Biomaterials, 28(9):1620-8 (2007)) have shown some improvements in compatibility in vitro. Coating metallic implants with bioactive molecules such as collagen (Muller, et al., Biomaterials, 26(34):6962-72 (2005)) and hyaluronan (Pitt, et al., J. Biomed. Mater. Res. A, 68(1):95-106 (2004)) has also improved cellular attachment and proliferation, though robustly adhering biomolecules to metal interfaces is not a trivial task. A common limitation to the clinical translation of materials functionalised with biomolecules is that the coatings are not robust enough to withstand in vivo exposure.
The requirements of a vascular device surface for immobilising proteins include high protein binding capacity, the ability to retain the protein activity and ultimately high biocompatibility. Proteins interact with metal alloys through non-specific, Van der Waals type interactions, resulting in protein-substrate association of variable strength. Proteins bound to a metallic surface in this way are often susceptible to removal by physical forces (e.g. by washing) and to unfolding and loss of activity. For medical applications, any bioactive coating requires a strong mechanism of attachment to create a surface robust enough to withstand in viva exposure, in this case, blood flow. Covalent immobilisation is usually preferred for this reason. A common approach is to construct a functional or biologically active thin film tethered to a supporting substrate. The biologically active molecules can be held in place by incorporation in Langmuir-Blodgett multilayers (Kahlert and Reiser, Cell Calcium, 36(3-4):295-302 (2004)), or on plasma treated polymeric surfaces (Gan, et al., Langmuir, 23(5):274′-6 (2007)) for example. Such methods however are usually not suitable for use with the metallic surfaces prevalent in cardiovascular applications.
The ability to covalently immobilise proteins onto metals is of interest for cardiovascular medicine as it enables the therapeutic modulation of the vascular biological properties of a large range of cardiovascular devices such as coronary stents. The immobilisation of a biomolecule requires reliable attachment and sufficient density to allow it to interact preferentially with cells and blood.
It is therefore an object of the present invention to provide materials which are suitable for vascular application that are biocompatible and not thrombogenic or have very low thrombogenicity.
It is a further object of the present invention to provide such materials which can withstand physiological stent conditions, including deployment, flow etc.